I AM concerned about how much they intend to flatten the power curve. If Orcs remain viable opponents for my level 10 character then there isn't a lot of space to ALSO make gnolls, bugbears, ogres, minotaurs, giants etc viable opponents for my level 10 character.
You don't have to flatten the power curve to make orcs viable at high level. You just have to flatten the attack/defense scale, and expand the hit point/damage scale to compensate.
Let's take 4E as an example. Consider a standard level 1 opponent, the goblin warrior, worth 100 XP. A five-person party at 1st level has an XP budget of 500 for an equal-level encounter. So they can take on five goblins.
Now consider the same party at 13th level, with an XP budget of 4000. In theory, this should mean they can take on forty goblins, right? But in practice, that encounter is a nightmare, because so much of the PCs' power gain is tied up in their higher attack and defense values. The PCs hit on anything but a 1, the goblins miss on anything but a 20. So the PCs hack away with their at-wills, laboriously grinding down the goblins' collective 1,160 hit points, while the goblins run around scoring an average of two hits a round for 1d8+2 damage apiece. Snoozefest.
Next, imagine that you keep the same relative power levels--five PCs to forty monsters--but instead of goblin warriors, you use orc warriors. Orc warriors are also worth 100 XP, but they're 9th-level minions instead of 1st-level regular monsters.
Suddenly you've got a good fight again. And the reason is that the 9th-level minions have comparable attack and defense bonuses to the PCs. The power differential comes from the fact that the minions have vastly fewer hit points than the PCs and deal less damage.
The proposal for 5E is this: What if we got rid of most of the attack and defense scaling, and instead relied on hit points and damage for the vast majority of level difference? Then you don't have to worry about the "forty goblins" situation. The goblin warriors will become
effectively minions (because PC damage has scaled to the point of being able to kill them in one hit). The relative power levels have not changed; a 13th-level PC is still eight times as strong as a 1st-level one, leaving just as much room for ogres and trolls and giants and such. But the mechanics producing those power levels are different.
(That said, I expect they will, in fact, flatten the power curve some as well. But you don't have to do that in order to get "orcs remain a threat at high levels.")